When parts are adhered, a heat curable type, an anaerobic curable type, a light curable type (e.g. ultra violet light or visible light), and combined type of adhesives are typically utilized. Among those, when the heat curable type adhesive is utilized to adhere optical parts, a heating process using an oven or similar is needed. As a result, the process generally takes relatively long time. In addition, some parts may not be heat resistant. When the anaerobic curable type adhesive is utilized, there are other restrictions due to characteristics of its curing process.
In view of such problems, ultra violet light curable type adhesive is generally utilized when parts (e.g. optical parts) are adhered. However, despite various advantages such as a quick action, a wide range adhering process, etc., the light curable type adhesives pose challenges due to shrinkage when cured with light (e.g. ultra violet light).
Such curing shrinkage is a serious problem when assembling is precisely performed. Specifically, a precise relative position can be disrupted due to curing shrinkage. Thus, curing shrinkage possibly impedes precision assembling.
As a countermeasure, four general types of methods are used. A first method suppresses curing shrinkage by thinning and decreasing an amount of adhesive to be coated as discussed in Japanese Patent Application Laid Open Nos. 2000-90481 and 10-309801. Japanese Patent Application Laid Open No. 2000-90481 discusses decreasing viscosity and pressure in order to uniformly coat a thin, low viscosity adhesive layer, so that a change in a volume of the adhesive, which is caused by a chance in temperature and curing shrinkage of the adhesive, can be slight and uniform. Japanese Patent Application Laid Open No. 10-309801 employs an intermediate hold member between an adhering member and adhering target to allow an adjustment margin. Thus, a thin adhesive layer is formed, so that a change in a volume of the adhesive, caused by curing shrinkage and a change in temperature, can be minimized.
However, Japanese Patent Application Laid Open No. 2000-90481 refers to a surface adhesion type and thereby needs special adhesive. Further, since Japanese Patent Application Laid Open No. 10-309801 restricts an adhesion structure and should employ indirect adhesion, the process needs additional parts and thereby undesirably increases the number of adhesion sections.
A second method improves uniformity of curing shrinkage by controlling irradiation of UV light and suppressing unevenness thereof as discussed in Japanese Patent Application Laid Open No. 2001-350072. Specifically, the proposal is to perform adhesion while maintaining a surface precision originally existing in optical parts by detecting unevenness of UV light intensity, and controlling a transmission light control section to suppress the unevenness so that uniformity of the curing shrinkage can be improved. However, in this method the adhesion process is restricted to a surface adhesion type, and displacement caused by curing shrinkage tends to unavoidably occur when adhesive is unevenly coated.
A third method suppresses displacement of parts caused by curing shrinkage by designing an adhesion structure and a process as discussed in Japanese Patent Application Laid Open No. 08-209075. However, this method also restricts an adhesion structure and is not a highly precise multipurpose UV adhering method.
Japanese Patent Application Laid Open No. 08-209075 designs structures of an adhering member, an adhering target, and an adhesion section, and then cures adhesive starting from that in the vicinity of two members simultaneously with coating so that a relative position between the two members would not be greatly affected by the curing shrinkage.
A fourth method, most widely employed, changes composition of the adhesive itself, as discussed in Japanese Patent Application Laid Open Nos. 10-121013, 07-201028, and 05-041408. Specifically, a technology of adding fine ceramic particles or filler to an adhesive so as to minimize its curing shrinkage has been known. Also known is a technology of adding thermal shrinkage plastic to affect curing times and shrinkage. In particular, Japanese Patent Application Laid Open No. 10-121013 adds filler having constant particle diameter and density so that a change in a volume of the adhesive caused by curing shrinkage and a change in temperature can be minimized. Japanese Patent Application Laid Open No. 07-201028 adds oxide ceramic fine particle average particle diameter no more than 10 micrometers in order to suppress a change in a volume caused by a change in temperature as well as curing shrinkage of the adhesive itself. Japanese Patent Application Laid Open No. 05-041408 employs ultra violet light curable type plastic including thermo shrinkage plastic and controls respective times when adhesive is cured by irradiation of ultra violet light and when shrinkage occurs due to heat so as to maintain the relative positions of the parts being bonded.
However, in such a situation, particular adhesive is required. Further, when the amount of adhesive is increased, the curing shrinkage correspondingly increases, thereby undesirable displacement of parts increases.